In recent years, semiconductor laser devices in which semiconductor lasers and modulators are integrated have been developed for application to optical communication. For example, a distributed feedback laser diode (hereinafter referred to as a DFB-LD) is dc-operated and laser light emitted from the DFB-LD is subjected to high-speed modulation in a light absorption modulator.
By the way, since a semiconductor laser is a forward biased device while a modulator is a reverse biased device, it is necessary to secure sufficient electrical isolation between the semiconductor laser and the modulator. Generally, in a semiconductor laser, its oscillation wavelength varies as the injection current is varied. In a semiconductor laser integrated with a modulator, though the laser is operated with a constant current, a modulation signal applied to the modulator and, through the isolation resistance, undesirably varies the current flowing toward the laser, whereby the oscillation wavelength of the laser varies. Because of the wavelength variation, the transmission waveform deteriorates during long-distance transmission through an optical fiber, so that the transmission distance is restricted. In order to improve transmission characteristics by solving the above problem, it is important to increase the isolation resistance.
FIG. 33(a) is a perspective view illustrating a semiconductor laser device integrated with a light modulator as a prior art semiconductor laser device (hereinafter referred to simply as a device L). FIGS. 33(b) and 33(c) are cross-sectional views taken along lines 33b--33b and 33c--33c of FIG. 33(a), respectively. In these figures, reference numeral 1 designates an n type InP substrate. The device L comprises a semiconductor laser (region I) having a diffraction grating (not shown) on a region of the semiconductor substrate 1 beneath an active layer 2, a modulator (region III), and an isolation part (region II) for separating the laser from the modulator. A mesa structure (optical waveguide) comprising the active layer 2, an n type InP cladding layer 1a, and a p type InP cladding layer 3 is disposed on the n type InP substrate 1. Fe-doped InP semi-insulating semiconductor layers 5 and n type InP hole trapping layers 6 are disposed on the n type InP substrate 1, contacting both sides of the mesa structure, i.e., the mesa structure is buried in these layers 5 and 6. A p type InP cladding layer 8 is disposed on the mesa structure and on the hole trapping layers 6. A p type InGaAs contact layer 9 is disposed on the cladding layer 8. An insulating film 10 and electrodes 11 are disposed on the contact layer 9.
In the device L so constructed, the laser (region I) has a diffraction grating under the active layer 2, and the diffraction grating enables the laser to produce stable single-wavelength light.
The active layer 2 of the laser (region I) and the active layer (light absorption layer) 2 of the modulator (region III) comprise a continuous InGaAs/InGaAsP multiple quantum well (MQW) layer. A difference in energies between the base level of the conduction band and the base level of the valence band in the quantum well layer is smaller in the laser than in the modulator. Therefore, when no bias voltage is applied to the modulator, light emitted from the laser is not absorbed by the active layer (light absorption layer) 2 of the modulator. However, when a reverse bias voltage is applied to the modulator, the laser light is absorbed by the active layer 2 due to the QCSE (Quantum Confined Stark Effect). That is, light emitted from the laser being dc-operated is modulated by varying a bias voltage applied to the modulator.
Further, the Fe doped InP semi-insulating semiconductor layer 5 and the n type InP hole trapping layer 6 are disposed on both sides of the optical waveguide structure comprising the active layer 2 and the upper and lower cladding layers 1a and 3, and these layers 5 and 6 serve as current blocking layers. Since Fe serves as a deep acceptor in InP, the Fe doped semi-insulating semiconductor layer 5 blocks electrons diffusing from the n type InP substrate 1. Further, the n type InP hole trapping layer 6 blocks holes diffusing from the p type InP cladding layer 8. Thereby, the threshold current of the laser is reduced and the efficiency of the laser is improved.
A method for fabricating the device L shown in FIGS. 33(a)-33(c) will be described using FIGS. 34(a)-34(c), 35(a)-35(c), and 36(a)-36(c).
Initially, a diffraction grating is produced in a region of the n type InP substrate 1 where the laser is to be fabricated. Thereafter, a cladding layer 1a, an active layer 2, and a cladding layer 3 are successively grown by crystal growth, preferably MOCVD (Metal Organic Chemical Vapor Deposition). Then, a stripe-shaped insulating film 4 having a width of 1.about.2 .mu.gm is formed on the cladding layer 3. Using the insulating film 4 as a mask, the structure is subjected to dry etching to form a mesa having a height of 2.about.3 .mu.m. This mesa structure M provides an optical waveguide.
Thereafter, as shown in FIGS. 35(a).about.35(c), a semi-insulating semiconductor layer 5 and a hole trapping layer 6 are grown at both sides of the mesa structure M so that the mesa is buried in these layers 5 and 6. Further, as shown in FIGS. 36(a)-36(c), after removal of the insulating film 4, a cladding layer 8 and a contact layer 9 are grown by crystal growth. Next, as shown in FIGS. 33(a)-33(c), a portion of the contact layer 9 opposed to the isolation part (region II) is removed. The range of this removal is 10.about.50 .mu.m along the longitudinal direction of the mesa structure M (33c--33c direction in FIG. 33(a)). Thereafter, an insulating film 10 is deposited, and portions of the insulating film 10, opposite regions where electrodes are to be produced, are removed. Finally, electrodes 11 are produced in contact with the contact layer 9, completing the device L shown in FIGS. 33(a)-33(c).
Although the waveguide (mesa structure M) of the device L is formed by dry etching, it may be formed by wet etching as shown in FIGS. 37(a)-37(c). FIGS. 38(a)-38(c), 39(a)-39(c), and 40(a)-40(c) are diagrams illustrating process steps for fabricating a device L in which wet etching is employed in formation of a waveguide. In this case, since isotropic etching is carried out when the waveguide is formed, the width of the insulating film 4 must be as wide as 5.about.8 .mu.m. Process steps other than the formation of the waveguide are identical to those already described with respect to FIGS. 34(a)-34(c), 35(a)-35(c), and 36(a)-36(c) for the case of employing dry etching and, therefore, do not require repeated description.
As described above, in the device L, sufficient electrical isolation is needed between the semiconductor laser (region I) and the modulator (region III). In the prior art structure, however, a high-frequency signal applied to the modulator leaks into the laser through the hole trapping layer 6 having a high carrier concentration and a low resistance.